DENVER — Three years ago, paleontologists were having the times of their lives, hoisting the bones of mammoths, mastodons and other now-extinct Ice Age species from the mud of a construction site near Snowmass Village as if carting off prized discoveries from a neighborhood garage sale.
In late October, some of those same scientists gathered for an afternoon in a darkened conference room in downtown Denver to share the insights they have developed after vacating the ancient lake bed in early July 2011.
Those insights won’t knock your socks off. Science can be a path small, even of doddering steps. However, as scientists synthesize the detritus of paleontological detail, a muscular story of great relevance today may yet emerge.
“It’s hard to get as excited about a squiggly line on a graph as with a mastodon femur coming out of the ground. No question,” says Jeffrey PIgati, a research geologist with the U.S. Geological Survey and team co-leader for the excavation. “But there is great information here nonetheless.”
Scientists have decided that a skeleton of a fossil deer from about 70,000 years recovered ago from Snowmass is about 10 percent larger than modern species and has a different dental arrangement, all of which argue for classification as a new species.
They continue to puzzle about placement of rocks amid mammoth bones that might suggest human presence somewhere between 40,000 and 70,0000 years ago. Also, there were parallel incisions into a bone. The result of natural, non-human processes? One expert thinks so, but not another.
“We don’t have an explanation for what this represents,” said Daniel C. Fisher, of the Museum of Paleontology and Department of Earth and Environmental Sciences at the University of Michigan.
It would be front-page news across the Americas if humans could be found in Snowmass before the start of the last advance of the glaciers. The consensus data of human arrival is 14,000 years ago, soon after the last glacial maximum. Several sites — one in Pennsylvania, another in Chile — suggest much earlier arrival of humans, but the evidence isn’t compelling.
The Snowmass evidence, intriguing though it is, also falls short of establishing humans in North America before the last glacial period.
A sharper look
Researchers have gained a sharper image of the climate of Snowmass between 135,000 and 45,000 years ago through such things as tiger salamanders, pollen from trees, and other organic matter that document the past climate.
Bookends for this time, called the Sangamon Interglacial, were the giant glaciers that descended the Snowmass Creek Valley. The first of the bookends, called Bull Lake glaciation, spilled over from the valley to create the unusual lake at the top of the moraine. That glacier started retreating toward Capitol Peak and Mount Daly 130,000 years ago. Glaciers started advancing again about 40,000 years ago.
In that expanse of 90,000 years, the climate was much like our own, but not exactly so. The beginning of that interglacial period was warmer. Sea level, for example, was 6 to 8 meters (20 to 25 feet) higher than today, as glaciers of Greenland and Antarctica had significantly melted. To see the warmth documented elsewhere in the type of animals and plants at Snowmass was not surprising.
Bones of at least 35 mastodons, who lived during this early, warmer time, have been identified. Some 18 tusks were recovered. Tusks can reveal much about the mastodons, even to the season of the year. For being a mastodon, it wasn’t a bad time to live. They found plenty of tree leaves and branches to munch on.
But how they died — that hasn’t been explicated clearly. Earthquakes have been ruled out. There was evidence of bones being punctured and gnawed on. But by whom and what? The bones of predators usually aren’t found along lake shores, only the prey. But were there short-faced bears, giant cats and others among the Ice Age predators?
What was surprising was the extent of cold from 90,000 and 100,000 years ago. Again, the coldness had been documented elsewhere. But at Snowmass, treeline descended 2,000 to 3,000 feet as the ancient lake at Ziegler Reservoir became alpine tundra.
“We know that it was supposed to be fairly cold during that time,” says Pigati. “It’s just a lot colder than expected based, for example, on the ice cores (of Greenland and Antarctica.)”
Then, temperatures warmed again. The fossil record at Ziegler Reservoir mostly ends at about 75,000 years ago. After that, there was a general cooling until the glaciers advanced again about 40,000 years ago.
Laura Strickland, from the U.S. Geological Survey, said most of the herbs, grasses and trees for much of this interglacial period were the same as those found today: chokecherry and red raspberry, potentilla and cinquefoil, pines and conifers. White fir and limber pine were also found in Ziegler Reservoir. Today, they are not. White fir, she said, is commonly found at lower elevations or in the southern part of Colorado.
Dane Miller, a botanist from the University of Wyoming, had studied cones from conifer trees and reported 20 “exquisitely preserved specimens.”
That exceptional preservation was also highlighted in remarks by Ian Miller, chair of earth sciences and curator of paleontology at the Denver Museum of Nature & Science and the co-director for the project. “Sedge and willow leaves are often still green, beetle parts are iridescent, and cones are often found still intact,” he said.
Aside from big bones, paleontologists have always thought the greatest value of Snowmastodon, as the site was dubbed, would be its ability to record the climate of the interglacial period.
The most complete picture of life forms and hence climate is gleaned from ice cores retrieved by drilling into glaciers in Greenland and Antarctica. Those ice cores go back 800,000 years and, in the case of Greenland, even detect the arrival of Romans in the British Isles, owing to increased traces of lead, the result of smelting.
There also are many paleontological sites in North America that provide glimpses into the past. But typically they provide one or two indicators, what scientists call proxies.
Ziegler Reservoir provides a richer view. It has scores of proxies, not just or two. Others are at 3,200 feet or lower in elevation. This one is at 9,000 feet. Even the dates of individual specimens are crisp, owing to the unexpected success of a dating technique called optically stimulated luminescence. Radio-carbon dating is useless beyond 40,000 years.
“We are looking into a window back into time that we had never been able to see through before,” says Pigati.